Hydrocracking is a petroleum refining process wherein the molecules of a hydrocarbonaceous material are partially decomposed in the presence of hydrogen to obtain a more desirable lower molecular weight hydrocarbon product. In recent years, the tendency of crude oil to become heavier and more difficult to refine is growing more significant day after day world wide. Meanwhile, the demand for middle distillates such as jet and diesel fuel is continuously increasing. This had led to a rapid development of hydrocracking technique. Much development work has been done into finding novel catalysts and improved hydrocracking conditions.
EP-A2-0093552 discloses a single stage hydrocracking process. The hydrotreating catalyst was arranged in a single reactor with the hydrocracking catalyst and ahead of it. The conditions used in the process include pressure of 5.25 to 7.0 MPa, temperature of 360 to 440.degree. C. and LHSV of 0.1 to 10 hr.sup.-1. The volume conversion to 345.degree. C.-products is 30 to 40 volume percent. The feedstock for the process is a heavy oil fraction having an initial boiling point of 200.degree. C. and normally of 340.degree. C. or higher such as coker heavy gas oil. The hydrotreating catalyst used in the process is a conventional denitrogenation catalyst and generally comprises one or more metals from Groups VIA and VIIIA of the Periodic Table supported on an amorphous, porous carrier such as silica, alumina or silica-alumina. The hydrocracking catalyst used in the process comprises one or more metals from Group VIA and VIIIA supported on a large pore zeolite such as zeolite X, zeolite Y or mordenite. One of the drawbacks of the process is that to achieve good distribution of the product of hydrocracking, the feedstock must be hydrocracked at limited conversion not greater than 50 volume percent to 345.degree. C.-distillate.
U.S. Pat. No. 4,686,030 discloses a process for mild hydrocracking a hydrocarbon feedstock containing nitrogen, sulfur components and feed components boiling it above 700.degree. F. (371.degree. C.) Typical mild hydrocracking conditions used in the process include temperature of 500 to 900.degree. F. hydrogen pressure of 200 to 1500 psig and LHSV of 0.05 to 3.0 hr.sup.-1. In the process between about 10 and 50 volume percent of the feed components boiling above 700.degree. F. are simultaneously converted to product components boiling at or below 700.degree. F. The catalyst used in the process is Mo--Ni--P supported on an amorphous porous refractory oxide and has a pore size distribution wherein at least 70 percent of the pore volume is in pores of diameter in the range from about 50 angstroms to about 130 angstroms and less than 10 percent of said pore volume is in pores of diameter greater than 110 angstroms.
U.S. Pat. No. 4,859,312 and EP-A2-0019959 disclose a process for producing middle distillates by hydrocracking (and isomerizating) heavy oils. But both processes require that the organic nitrogen content of the feed be less than 50, preferably less than 10 ppmw.
The present invention therefore provides a process for producing middle distillates by hydrocracking a heavy distillate oil using novel hydrofining catalyst and hydrocracking catalyst. The process can be operated at middle pressure with once-through volume conversion of up to 80 volume percent, is suited to treat a poor-quality feedstock with the nitrogen content up to at 1500 ppmw and the nitrogen content of the charge to be contacted with the hydrocracking catalyst can be allowed up to 150 ppmw.